Testing spatial uniformity of the CR spectrum in the local ISM with gamma-ray observations

TL;DR

This study uses Fermi LAT gamma-ray data to examine the cosmic ray spectrum in local interstellar structures, finding evidence for spectral uniformity across different regions of the Galaxy.

Contribution

It provides the first comparative analysis of CR spectra in multiple local ISM structures, supporting the hypothesis of spectral uniformity in the Galaxy.

Findings

CR spectrum in Aquarius HI shell matches previous observations

Assumptions on diffuse background influence CR spectral shape in some clouds

Evidence supports uniformity of CR spectra in the local Galaxy

Abstract

Gamma-ray observations of nearby radio-line-emitting gas structures in the interstellar medium allow us to probe the spectrum of cosmic rays (CRs). In this paper, we analysed Fermi Large Area Telescope (LAT) gamma-ray observations of three such structures located near each other to check if their CR spectra are compatible with that of the CR background or might provide evidence for a population of "fresh" CRs. We found that the shape of the gamma-ray spectrum in the Aquarius HI shell is consistent with the previously published stacked gamma-ray spectrum of the Gould Belt molecular clouds. We also found that assumptions on the diffuse Galactic gamma-ray background affect the spectral shapes of CRs derived in the R Coronae Australis and rho Ophiuchi molecular clouds in which spectral deviations had previously been suggested. These two facts provide evidence to support the hypothesis of uniformity of the shapes of cosmic ray spectra in the local Galaxy environment.